Factor out {SharedSingleton,ImmutableOffset}MapTemplate

Factor out {SharedSingleton,ImmutableOffset}MapTemplate

Exposing the single-entry map case has the advantage of providing
instantiateWithValue(Object) method, side-stepping the need for
a temporary array. Split it out of ImmutableMapTemplate, so that
users can provide customized handling as needed.

The same holds true if the keyset is known to contain more than
one key, in which case using ImmutableOffsetMapTemplate can be
faster.

JIRA: YANGTOOLS-917
Change-Id: If08f492ff59bc89f3826a4122cfbcf34657c2210
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit bd16b6161802322cb33c39642ddeb8a59b3e5d48)